The present invention relates generally to the qualitative and quantitative analysis of liquids and, more particularly, to the use of time resolved spectroscopic analysis of the emitted electromagnetic radiation from the liquid under investigation following two consecutive dielectric breakdown events therein caused to occur in the same volume.
Laser generated sparks in air have been used to detect the presence of atoms in vapors, aerosols, and particles. In the high temperatures of the spark plasma, material is reduced to elemental form and excited. Emitting species (ions, neutral atoms, and simple molecules) are identified by spectrally and temporally resolving the spark light.
Dielectric breakdown of pure water and several organic liquids occurs with focused laser pulse powers of 10.sup.10 -10.sup.11 W/cm.sup.2. However, the breakdown threshold is influenced significantly by the presence of particles or dissolved materials. By contrast, breakdown of air occurs with power densities as low as 10.sup.7 -10.sup.8 W/cm.sup.2. The higher laser pulse intensity requirements for producing dielectric breakdown in liquids can be met with commonly available lasers, rendering laser spark spectrometric analysis applicable to situations requiring a real-time and/or noninvasive analysis procedure. Real-time analysis is possible because the laser spark both prepares (vaporizes) and excites the sample. The technique is noninvasive in the sense that only optical access to and from the sampled medium is required. Laser sparks have been generated in liquids using 1-2 J pulses of 10-30 ns duration. Moreover, stable sparks can be produced using tightly focused laser pulses of 40-50 mJ at powers of about 3 MW, which is well within the specifications of reliable and small commercially available lasers which can operate at 10 to 20 Hz.
Mention of multiple laser pulses for time and spatially resolved spectrometric observations of laser generated plumes from the surface of a solid aluminum alloy appears in "Q-Switched Laser Energy Absorption in the Plume of an Aluminum Alloy," by E. H. Piepmeier and H. V. Malmstadt, Anal. Chem. 41, 701 (1969). Therein the authors discuss the investigation and modeling of such plumes and teach the use of time resolution and spatial resolution of laser spark emissions to separate sample spectra from spectra of the atmospheric species which surround the sample under investigation. The authors point out that a significant amount of laser energy is absorbed by the plume and therefore a multispike laser pulse would be a good way to further excite the sample species once they have entered the plume. Although this concept has similar motivation to the subject invention, no mention is made by Piepmeier et al. of any comparison in signal-to-noise characteristics with the results of a single spark at the surface of the object and multiple sparks within the laser induced plume. In fact, no observations are reported at the surface of the sample. The substantial absence of Al I species above the surface is only temporary, there being a transfer of atoms from the surface to points above the surface shortly after the spark occurs. This implies that atoms are being vaporized by the laser spark in significant numbers at the surface of the sample, many of them almost certainly in excited electronic states. Therefore Piepmeier et al. teaches away from the subject invention in that the analytical measurements are performed in the region of the laser generated plume with the intention of investigating its properties to improve laser sampling techniques rather than those of the surface at which the spark takes place. Our invention, on the other hand, teaches spark formation inside of a liquid and away from atmospheric presence so that the effects of such complications can be ignored. Moreover, substantial improvement in signal-to-noise ratio occurs, individual line widths narrow and line intensities increase when subsequent sparks are generated in the volume viewed in the subject invention over that resulting from the application of a single spark.
In "Multifrequency Laser Breakdown in SF.sub.6," by P. J. Hargis, L. C. Pitchford, T. A. Green, J. R. Woodworth, and R. A. Hamil, an abstract of a talk presented at the 35th Annual Gaseous Electronics Conference, Dallas, Texas, October 19-22, 1982, the authors describe the application of two laser pulses of different wavelengths and coincident in space and time to a gaseous sample of SF.sub.6 in order to enhance the intensity of the breakdown spark therein. The spectrum of the enhanced spark showed an increase in atomic fluorine emission. The wavelengths utilized in our invention do not significantly affect the measurements. The emission. Moreover, Hargis et al. do not mention time resolution of the electromagnetic emission, or the use of the multifrequency laser breakdown for analytical purposes. In fact, the emphasis of this work is for high voltage switching.
Use of laser-induced breakdown spectroscopy (LIBS) for detection of contaminants in liquids was described in a paper presented orally at the 24th Rocky Mountain Conference, Denver, Colorado, August 1-5, 1982 and the abstract therefor published on July 15, 1982. In the abstract for "Direct Detection of Contaminants in Liquids via Laser-Induced Breakdown Spectroscopy," by D. A. Cremers, L. J. Radziemski, and R. J. Martinez, the authors disclose the application of LIBS to liquid samples. However, no mention is made of the double spark technique of the instant invention.
Accordingly, an object of the subject invention is to significantly enhance the emission spectra of species excited by laser sparks in liquids over those obtained by conventional LIBS techniques.
Another object of our invention is to improve the detection limits of species in liquids over those obtainable by conventional LIBS techniques.
Yet another object of the instant invention is to monitor species in liquids which cannot be observed with the single spark LIBS technique.
Still another object of our invention is the resolution of closely spaced emission lines from species present in liquids following excitation by means of a laser spark.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.